1. Field of the Invention
The present invention relates to a ferrous object sensor assembly capable of end on sensing of a ferrous object including a tooth or notch on a ferrous wheel or vane at zero speed and immediately upon powerup and, more particularly, to a ferrous object sensor assembly comprising a magnetic flux responsive sensor having a sensing plane which provides an electrical output signal which varies as a function of the change in magnetic flux density applied to the sensing plane. The ferrous object sensor is disposed adjacent a lateral surface of a magnet, intermediate opposing pole faces and is responsive to radial magnetic flux components emanating from the lateral surface.
2. Description of the Prior Art
Some known magnetic flux responsive sensors, such as Hall effect sensors, are used as proximity sensors. Such sensors are used to detect the presence or absence of a magnet, for example, as illustrated in FIG. 1. These sensors rely on the change of magnetic flux density applied to the sensing plane of the sensor.
Other known Hall effect sensors are used in a wide variety of applications to detect the presence of a tooth or notch on a ferrous vane wheel, such as shown in FIG. 2. Such sensors are incorporated into assemblies and used, for example, in the automobile industry for breakerless ignition timing sensing circuitry. In this application, the Hall effect sensor 30 is mounted on a carrier 38. A magnet 34 is mounted a fixed distance from the sensing plane 32, defining an air gap 36 and forming an assembly. In this application, a ferrous vane wheel 40 having one or more teeth 42 is rotatably mounted with respect to the air gap 36 so that the teeth 42 pass through the air gap 36 when the ferrous vane wheel 40 is rotated. When a tooth 42 is disposed in the gap 36, the magnetic field is shunted away from the sensor 30 changing the magnetic flux density applied to the sensing plane. This ferrous vane wheel principle is best understood with reference to FIGS. 4 and 5. FIG. 4 represents the magnetic flux pattern of the assembly, illustrated in FIG. 2, across the air gap 36. In the absence of a tooth 42, the magnetic field is applied directly to the sensing plane surface 32 of the sensor 30. FIG. 5 represents a condition where a tooth 42 is disposed between a pole face and the sensing plane 32 of the sensor 30. In this situation, since the reluctance of the tooth 42 is much less than the reluctance of air, the tooth 42 shunts the magnetic field away from the Hall effect sensor 30.
The Hall effect sensors used in the applications shown in FIGS. 1 and 2 operate as a function of the change in magnetic flux density. In other words, the Hall effect sensor is responsive to a predetermined level of magnetic flux density at a predetermined polarity. When such a level is present, the Hall effect sensor will provide an output voltage signal representing either the presence or absence of a permanent magnet body (FIG. 1) or of a ferrous body (FIG. 2). Accordingly, such Hall effect sensors are typically used for proximity sensing.
The Hall effect sensor, illustrated in FIG. 1, relies on a moving magnet with respect to a Hall effect sensor. Such an approach is relatively inconvenient and may present various technical difficulties. For example, in order to use such sensors to detect rotating members, a series of magnets would have to be mounted on the periphery of the rotating members. This would be relatively expensive and inconvenient. An application, such as illustrated in FIG. 2, requires relatively precise alignment of the ferrous vane wheel with respect to the air gap to prevent collisions between the ferrous vane wheel, the magnet and the sensor.
FIG. 3 represents another known application of a conventional Hall effect sensor used to detect teeth or notches on a ferrous wheel. In this application, one pole face of a magnet 34 is disposed adjacent the Hall effect sensor 30 forming an assembly 44. The assembly 44 is rigidly mounted with respect to the ferrous wheel 43. As the individual teeth 45 pass near the sensing plane 32 of the sensor 30, the magnetic flux varies. More specifically, as a tooth 45 passes adjacent the sensing plane 32, the magnetic field density is increased through the sensor 30. When a notch 61 (or area between contiguous teeth 45) is disposed adjacent the sensing plane 32, the magnetic field density applied to the sensing plane 32 decreases. Such a Hall effect sensor 30 is thus able to provide an output voltage signal representative of the presence of a tooth 45 or notch 61.
However, when used as in FIG. 3, the small output signal values available from Hall effect sensors (and MRE's) often make it desirable to further amplify the output signal so that the sensor assembly provides a final output signal of high signal to noise ratio. Such an amplified output signal mitigates the need for specialized filtering circuits or special transmission cable, thus potentially reducing overall system cost. Therefore, such known Hall effect sensors, such as found in a Sprague type UGN-3503U, are capacitively coupled to an amplifier which boosts the output voltage signal to a much higher level. Since coupling capacitors are regulated by a resistive capacitive time constant, the output of such a Hall effect sensor assembly provides an output voltage signal representative of the change in magnetic flux density with respect to time. Thus, such Hall effect sensor assemblies cannot be used to detect a tooth or notch on a ferrous wheel at either zero speed or very high speeds. In such an application, for proper operation, the speed of the ferrous wheel would depend on the capacitor time constant. Accordingly, such Hall effect sensor assemblies cannot be used as proximity sensors.
All three Hall effect sensor applications, illustrated in FIGS. 1-3, rely on pole face magnetism. In other words, the magnetic flux emanating from a pole face is used to provide the required magnetic flux density to actuate the Hall effect sensor. However, it is known that at the pole face, temperature induced magnetic flux density change is significant. Thus, such devices can provide erratic output voltage signals when used in an environment subject to a wide range of temperatures, such as an automobile transmission or engine.